The best way to prove if a team has an open differential or not should be to lay out a cone slalom on the front stretch of any given race track.
Have the offending team attempt to drift the course. If done poorly, open diff. If executed perfectly everyone applauds, raises their beers in salute and we all go home.
(Neon green text)

I've been racing in ChampCar for a just little while.
I think the number one key to success I could share from my experience would be,
"Always give yourself an out."
When committing 110% to something, regardless of fault, be ready to accept the consequences.

I was working for Formula Experiences on Saturday on the skidpad being teased by the race going on.
So, I asked @Huggy if I could drive with them Sunday. He said yes and I was like "Party on Wayne."
I hopped in the car for 2nd stint Sunday. 1 lap down to the leader #346.
I unlapped ourselves in the first 15 minutes or so. Then passed them for the lead 45 minutes later before lapping them again close to the end of my stint.
I enjoyed the full wet to partially wet conditions, Huggies car handles it well and was very pleasant to drive. My last race with the team was last year at this event which we won in the 146. It was nice to come back right to winning form with the 246 and get the car's 1st victory!
Thanks to everyone on the team, and everyone on and off the track.

Sure, that's paraphrasing for,
The driver could have strategically yielded to me and other cars around me and had overall maintained a higher average speed.
They didn't do anything wrong, just an observation.